As is well known a conventional bottle opening is rimmed by a bead of slightly larger diameter than the neck of the bottle immediately below it and a crown seal comprises a metal cap which, in the finished, closed condition, has a skirt, portions of the lower margin of which are pressed under that bead to secure the cap in place. Usually the top of the cap has a resilient layer or annulus applied to its under surface to make hermetic contact with the top of the bottle.
In the industrial application of crown seals a blank is first prepared which is similar in many respects to the finished cap but with its skirt more flared than it is in the finished cap.
The skirt of the blank is corrugated or formed with spaced apart radially extending ribs in it. Those ribs usually decrease in depth towards their radially outer ends and also decrease in width in the same direction.
The blank is applied to the bottle by means of a forming tool which is basically cylindrical in shape and which is brought down so as to contact the flared skirt of the blank after the blank has been positioned on the top of the bottle. The forming tool bears against the aforesaid ribs, which because of their shape are relatively stiff and as the forming process continues those ribs are bent about the line of junction between the flared and unflared portions of the skirt so as to conform with the cylindrical inner surface of the tool, whilst the portions of the skirt intermediate each rib are caused to move under the bead and secure the cap tightly in place.
With prior known crown seals the finished ribbed skirt of the cap displays relatively sharply edged protruberances and it is uncomfortable or maybe even painful if it is grasped tightly in the hand, for example, in the crotch between the thumb and first finger.
This is a problem of some magnitude in the case of relatively newly developed twist-off crown seals. In such seals, the finish of the bottle is formed with a multi-start thread superimposed on the rim bead and the skirt of the cap is pressed about that thread formation during the application of the blank to the bottle. Thus, if the cap is subsequently turned by hand the effect of the thread formation is to expand the skirt and at the same time force the cap upwardly off the bottle. This necessitates a tight gripping of the cap and consequent discomfort is apt to occur.